1. Field of the Invention:
The invention relates to a process for recording, storing and displaying optically readable information, using a suitable plastic material as a storage medium.
2. Description of the Related Art:
In general, different polymer species are mutually incompatible because of the low entropy of mixing of the long polymer chains and the positive mixing energy between polymers. The finding that, contrary to the general tendency, certain polymer systems exhibit miscibility, therefore calls for a theoretically justified explanation of the phenomena.
For example, polyvinylidene fluoride is miscible with polymethylmethacrylate (PMMA) and with polyethylmethacrylate (U.S. Pat. Nos. 3,253,060, 3,458,391 and 3,459,843). To explain this phenomenon, a concept was developed, according to which specific interactions between groups of complementary dissimilarity in two polymers are said to make a positive contribution to the heat of mixing. This concept appeared to be suitable as a criterion for selecting other, miscible, binary polymer systems (D. R. Paul et al., Polymer Engineering & Science 18 (16) 1225-1234 (1978); J. Macromol. Sci. - Rev. Macromol. Chem. C 18 (1) 109-168 (1980); Kirk-Othmer, 3.sup.rd Ed. Vol. 18, pg. 443-478, J. Wiley, 1982).
The former literature reference also quotes a listing of known miscible polymer systems. The glass transition temperature T.sub.g of the mixtures was frequently used as proof of miscibility.
The occurrence of a lower critical solution temperature (LCST) is used as a different test of the miscibility of polymers. The existence of the LCST is based on the fact that a previously clear mixture separates into phases and becomes optically cloudy on being heated. This behavior represents unambiguous proof of the fact that the original polymer mixture consisted of a single, homogeneous phase at equilibrium.
In some systems, demixing is a reversible process on cooling. Up till now, LCST behavior has been reported for the following polymer systems: PMMA/styrene-acrylonitrile copolymers; polystyrene/polyvinyl methyl ether; poly(.epsilon.-caprolactone)/sytreneacrylonitrile copolymers; chlorinated rubber/ethylene vinyl acetate copolymers; PVC/ethylene vinyl acetate copolymers; poly(.epsilon.-caprolactone)/polycarbonate; polyvinylidene fluoride/PMMA; polyvinylidene fluoride/polyethylmethacrylate; polyvinylidene fluoride/polymethylacrylate; polyvinylidene fluoride/polyethylacrylate; polyphenylene oxide/o-chlorostyrene-p-chlorostyrene copolymers; polystyrene/polycarbonate of tetramethylbisphenol A; polyvinyl nitrate/polymethylacrylate (D. R. Paul et al., loc. cit.), as well as for the polymer systems PVC/poly-n-hexylmethacrylate, PVC/poly-n-butylacrylate, poly-n-propylacrylate (D. J. Walsh and J. G. McKeown, Polymer 21, 1330-1334 (1980), as well as for chlorinated polyethylene/butyl acrylate (D. J. Walsh et al., Makromol. Chem 184, 1459-1468 (1983)) and for PMMA/chlorinated polyethylene (D. J. Walsh et al., Polymer 23, 336-339 (1982).
The technique has followed various paths in order to produce optically perceivable information by the use of heat (or of energy, which can easily be converted into heat) ("Thermography").
In DE-OS No. 2,328,900 a thermographic copy material is described.
With this material, thin films of homogeneous polymer complexes of proton donor and acceptor units, which contain bonds dissociating at temperatures ranging from 50.degree. to 120.degree. C., form a thermally latent image. The image can be developed by moistening or spraying with a dye solution, a solvent or a gas (ammonia).
From DE-OS No. 3,042,331, a thermographic recording material is known, which contains a silver compound that is not light sensitive, in conjunction with an impact-resistant acrylic resin. The silver compound is sensitized by heating in the absence of light.
A layered, thermographic recording material is also the object of Japanese Patent No. 75/128,528 (cf. Chem. Abstr. 84, 128762b). With this material, visible images are obtained, when heat or other forms of energy, which, like light and electricity, can easily be converted into heat, are allowed to act on the layered material. An imaging layer of at least one fluorinated hydrocarbon and a compound selected from the group consisting of oximes, betaines, organometallic compounds, complex compounds of transition metals with aromatic compounds, guanidines and their salts, urea, thiourea and its salts, is applied to a carrier. For example, a dispersion of a liquid acrylic resin of a polymeric fluorinated hydrocarbon with isobutyroketone and toluene and acetaldoxime is applied on a high-grade paper support. On inscribing with a pen heated to 200.degree. C., the inscribed areas appear black.
From U.S. Pat. No. 4,307,942, a device is known for preventing entrance of the sun's rays above a given temperature. This device consists of a layer of a porous polymeric material, a solvent and a temperature-sensitive substance, with which the porous material is impregnated and which has a negative solution entropy in said temperature range.
Such technical applications of polymers for automatically regulating energy penetration are quite rare. Solutions of individual polymers, which form a completely transparent solution or a transparent gel below a certain temperature range, but, above this temperature range, react with precipitation of small solid particles, which have a refractive index that differs from that of the solution, so that the solution loses about 50% of its transparency for visible light, are recommended in U.S. Pat. No. 4,307,942 as a non-mechanical sunshade.
As already mentioned, a relatively small number of mutually compatible polymer has been known up till now. Some of these combined systems exhibit the phenomenon that a cloud point occurs on heating; in other words, as the system is heated, a temperature is reached, at which this polymer mixture breaks up once again into two incompatible polymer systems. These systems show a lower critical solution temperature (LCST). FIG. 1 shows the phase diagram of such a polymer mixture with a lower critical solution temperature.
Compatible polymer mixtures, which have an LCST, have until now been investigated strictly from a scientific point of view. Utilization of this phenomenon to solve defined, technical tasks has, so far, not been proposed. The reason for this may well be that the cloud points of previously known compatible polymer mixtures with an LCST were observed only at very high temperatures, which frequently already caused decomposition of the individual polymers. For example, the PMA/polyvinylidene fluoride (PVDF) system shows a cloud point at about 300.degree. C. and the polyethylmethacrylate/PVDF system shows a cloud point of 240.degree. C.; PMMA/styrene-acrylonitrile copolymer has a cloud point at 170.degree. C. and polycarbonate/polycaprolactone has one at 260.degree. C.
Displaying optically perceivable information under the action of thermal energy was previously based predominantly on chemical conversions or on the utilization of phase transitions in liquid crystals.
A display of optical information, which comes about due to the fact that an optically clear storage medium loses its transparency in a defined manner, e.g. in specific geometric regions, under the action of thermal energy and appears white, for example, was until now unknown. A reversible transition between the transparent and the cloudy, that is, the white state of the storage medium, would appear to be particularly interesting.
Accordingly, new ways of displaying optical information in which an optically clear storage medium loses its transparency in a defined manner by using thermal energy, are of interest.